1. Field of the Invention
The invention pertains generally to optical fibers with high bandwidth and low attenuation and is more particularly directed to such optical fibers which can be easily terminated with a simple crimp and cleave connector process.
2. Description of Related Art
Optical fiber has revolutionized the communications industry in providing a low cost high bandwidth transmission medium for all types and forms of information. An area in optical fiber communications systems where improvements are continuing to be made is in the connectorization of such fibers. Because the optical fibers of these systems are essentially made of a silica glass and the diameters of the fiber are small, it is essential to put a connector on the ends of the fiber for the purpose of joining two pieces of fiber together or plugging the fiber into optical transmitter and receiver. Connectors can be put on the fiber by fusion splicing the fiber to a fiber pigtail (a short piece of fiber with connector pre-assembled in a factory) or by epoxying the fiber to the connector, then scribing and polishing the end. Such methods have been successful in the past, but are relatively complex, time consuming and expensive. For future planned optical distribution systems, such as fiber to the home systems, there are increasing requirements for the number of field terminations. Fast and easy connectorization becomes more important.
Low cost connectors for polymer clad, multi-mode optical fibers have been developed that rely on a simple crimp and cleave process. The optical fiber is cleaved by scoring the glass fiber with a fracture line and then breaking it so that an even plane orthogonal to the longitudinal axis of the fiber is formed across its end. The ends of two such cloven multi-mode fibers can then be joined by abutment, with or without an adhesive or index matching composition. The mechanical portion of the connector that supports the splice and holds the optical fiber ends together is then crimped to the outer surface of the optical fiber by deforming specially adapted (typically metal) spacing rings or features which then grip that outer surface.
One multi-mode fiber which is particularly adapted to the crimp and cleave connectorization process is a step index (SI) optical fiber having a non-doped silica core and an optical polymer coating of a lower refractive index forming a cladding layer. This optical fiber then may be coated with another buffer layer of a tough polymer such as ETFE without optical qualities for protection and to receive the deformation of the crimping mechanisms of the connectors. The first polymer coating is made in accordance with the teachings of U.S. Pat. Nos. 4,707,076 or 4,511,209 issued to Skutnik, et al. and comprises at least one ethylenically unsaturated monene, a polyene and a curing initiator. The disclosures of Skutnik, et al. are hereby incorporated by reference. Further additions are made to the polymer coating to make it hard so that during the cleaving process it will fracture cleanly with the glass core but still withstand the crimping process without breaking. This optical fiber exhibits many of the desirable optical qualities that are needed in the distribution type communications systems including its large core (typically about 200 μm), high numerical aperture (typically about 0.37) and the ability to be connectorized by the crimp and cleave process. However, because of its low bandwidth-length-product (BLP), approximately 10 MHz-km., it is difficult to use such optical fiber in newer applications which may require higher speeds or longer fiber link lengths.
Increased bandwidth can be realized in a multi-mode optical fiber by changing the index profile of the core from a constant (SI) to a graded index (GI), usually by creating a profile that follows the power function Rα, where α is generally between 1.5 and 3. This is generally accomplished with an optical fiber having a silica core by doping the core material with a refractive index changing material as is conventional in the art. Increasing the refractive index of silica is accomplished by the addition of Germanium (Ge) or other known refractive increasing elements to the silica but is expensive in such large core fibers. However, with the large silica cores of the prior optical fibers used in crimp and cleave connectors, the addition of Ge to the entire core is prohibitive because of its cost and will not yield a sufficient increase in BLP for the fiber.
Therefore, there is a need for a large core optical fiber have a large numerical aperture and increased bandwidth-length product which is also inexpensive to manufacture and can be joined by a crimp and cleave connectorization process.